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136

SOME PROBLEMS OF FATIGUE

OF BOLTS AND BOLTED JOINTS

IN AIRCRAFT APPLICATIONS

LEONARD MORDFIN

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

WW n
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

technical Ntote

136

JANUARY 1962

SOME PROBLEMS OF FATIGUE

OF BOLTS AND BOLTED JOINTS

IN AIRCRAFT APPLICATIONS

Leonard Mordfin

Report to Bureau of Weapons

Department of the Navy

on

Interdepartmental Government Orders 17-60-7009-WEPS

(BuWeps Project TED NBS RAAD-23-001) and

19-61-8001-WEPS.

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient of limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price $1.25

CONTENTS

Page

SUMMARY 1

1INTRODUCTION 1 2. EFFECTS OF THE PRELOADING OF BOLTS ON THE FATIGUE

STRENGTH OF BOLTS AND BOLTED JOINTS 3

3

FATIGUE OF BOLTS 6

3 .Shape of Bolt Threads .- 6 a

Root radius 6

b Flank angle 6

c

Depth of engagement 7 d

Pitch 7 e

Taper 7 f

Lok-Threds 7

3 .

Thread Forming 8

3.3 Improving Fatigue Strength at the First Thread 9

3.4 Effect of Nut Design 11

3.5 Bolt Head and Shoulder Design 12

3.6 Bolt Materials 14

3.7 Platings 15

3.8 Civil Engineering Bolts 16

3.9 Cap Screws 17

4

FATIGUE OF BOLTED JOINTS 17

4 .

Joint Configuration 18

a. Single shear joints 18

b

Double shear joints 18

cScarf joints 19

4 .

Joint Proportions 19

4.3 Bolt Pattern 20

4.4 Preloading of Joints 21

4 .

Hole Preparation 23

4.6 Fretting 25

5. EFFECTS OF TEMPERATURE ON THE FATIGUE STRENGTH OF

BOLTS AND BOLTED JOINTS 25

5.1 Bolts 26

5 .

Nuts 27

5.3 Platings 28

5:4 Joint Materials and Designs 28

6

DISCUSSIONS OF THE FATIGUE OF BOLTS 29

6.1 On the Evaluation of Bolts for Various Applications.. 29

a. Bolts for tension fatigue applications 30

b. Bolts for non- tension fatigue applications 31

c. Bolts for elevated temperature fatigue

applications 31

111
6.2 On the Specification of Fatigue-Resistant Bolts 32

a. Bolts for use at ordinary temperatures 32

bBolts for elevated temperature use 32

6.3 On Errors in Fastener Replacement 34

a. Decrease the chances of installing a wrong bolt... 34

b. Decrease the chances of failure resulting from

the installation of a wrong bolt 35

c. Increase the ease with which the presence of an improper bolt in an assembly may be detected... 35

REFERENCES 36

IV
SCME PROBLEMS OF FATIGUE OF BOLTS AND BOLTED

JOINTS IN AIRCRAFT APPLICATIONS

by

Leonard Mordfin

SUMMARY

The profuse variety of aircraft bolts which

is available has made the evaluation and specification of bolts for engine and structural use ex- tremely complex, particularly insofar as fatigue

and hot fatigue environments are concerned. The

state of knowledge of fatigue of bolts and bolted

joints is surveyed and critically appraised in

terms of aeronautical practices. Using this material as a basis , recommendations are made regarding the evaluation and specification of aircraft

bolts for fatigue situations and regarding the

growing problem of errors in fastener replacement.

1 . INTRODUCTION

The life expectancy of aircraft bolts Is generally comparable to

that of the fatigue-sensitive structures in which they are employed.

Nevertheless, the Aerospace Industries Association estimates that the

use of bolts in aircraft joints will decrease by about two-thirds over

the next ten year period. There are many reasons for this anticipated

trend; one of the most significant stems from the fact that the selection

of the proper bolt for the severe environments associated with hypersonic

flight is exceedingly complex.

To a large extent, the rapid development of aircraft bolts represents a tribute to the manufacturers, whose advances during the last

decade have been considerable. Many of these advances, however, have

been made at the expense of standardization since bolts for special

purposes can be made far stronger than bolts for general use. This,

together with the fact that ours is a competitive economy, has resulted

in the manufacture of a staggering number of different bolt types, each

costing considerably more than its general purpose antecedent. When the

designer, specifier or purchaser is confronted with this situation,

knowing that the bolts were developed primarily (although not exclusively) for greater static strength, his task of specifying bolts for

particular fatigue applications becomes rather formidable.
- 2 -

A project was undertaken, therefore, to survey available knowledge

on the fatigue properties of aircraft bolts, in the hopes that with this

foundation, answers could be found for many of the perplexing questions

which arise when attempting to prepare specifications for bolts to be

used in fatigue applications

While the above survey was being carried out, another investigation

was in progress, the first phase of which was set forth as "... a survey

on structural fatigue at elevated temperatures to establish current

requirements and thinking in this area. Such survey shall be an extension of work previously performed*... The materials, fasteners, and

configurations which are currently used, and are likely to be used, for

skin-to-spar cap type connections in the heat-affected zones of supersonic aircraft shall be determined..." The purpose of this phase of

work was to provide a basis for designing suitable specimens for a test

program.

For obvious reasons, it was considered desirable to incorporate some

of the results of the latter survey into the present report, particularly

with regard to general design principles for increasing the fatigue

strength of joints.

The purpose of this report, then, is to present a survey of available knowledge on the fatigue of bolts and bolted joints, and to use

this information to discuss various aspects of the general problem of

specifying bolts, materials and configurations for aircraft fatigue

applications

The survey itself turned out to be a more Herculean task than was originally anticipated. It is estimated that well over a thousand

papers, reports and articles have been written on the subject. Fortunately, a number of excellent surveys have already been prepared (refs.

2 through 8) which cover most of the pertinent material written prior to

1955. The present effort, therefore, was largely devoted to reviewing

the wealth of material written since that time. To this end, approximately 300 publications and reports were studied. As might be expected

in so prolific a field, the duplication is sizeable. For this reason,

it is felt that little would be accomplished by the listing of a complete

bibliography. The following referencing system has, therefore, been

adopted. Refs. 2 through 8 shall be considered basic and applicable to

this entire report. In addition, several supplementary references are given for each section or subsection of the text. These are intended

merely to introduce the reader to further information on the subject

and actually represent only a fraction of the material which is reviewed

in the text

*See reference 1
- 3 -

In presenting the results of the survey, it was considered desirable

to divide the subject into numerous sections and subsections. Considerable overlap exists, but this is unavoidable. Two main sections are devoted, respectively, to fatigue of bolts as such, and to fatigue of

joints. These are further divided into subsections dealing with various

aspects of the problems. Although the author has taken the liberty of

inserting his comments throughout the report, a third main section has

been reserved exclusively for this purpose. Two more main sections are devoted to the effects of preloading of bolts and to temperature effects

since these apply largely to both bolts and joints and could not be

treated fully under either of the latter categories. The reader is re- ferred to the table of contents for an outline of the report prior to

further study.

Much of the material presented deals with means of improving the

fatigue strengths of bolts and joints. In some cases this material is

based on a limited amount of experimental evidence. The designer is

cautioned, therefore, to regard this information as introductory rather

than as directly usable data unless suitable check testing is done.

2. EFFECTS OF THE PRELOADING OF BOLTS ON THE FATIGUE

STRENGTH OF BOLTS AND BOLTED JOINTS

(Supplementary references: 9 through 16)

The most important factor among those which influence the fatigue

strength of bolts and bolted joints is preloading of the bolts . The

ability to preload bolts by the application of a high torque makes it

possible to increase the fatigue strength of a bolted joint over that

of an otherwise comparable hot or cold riveted joint.

The application of a preload to a bolt raises its mean stress but,

for a given applied dynamic load, it reduces the effective cyclic stress

amplitude felt by the bolt. In most bolting steels, fatigue life is

more sensitive to stress amplitude than it is to mean stress, and hence

fatigue life can be greatly increased by preloading. Increases of 50 to

250 percent are, in fact, common. Also, scatter is notably reduced. The

benefits of preload increase with the amount of preload, and use of preloads up to 90 percent of the yield strength of the bolt are generally

desirable. In no case, however, should the magnitude of the preload be

sufficient to indent the joint material or, in combination with the

external loads, to create a crack at the thread root.

Stress analyses* reveal that, for a given preload, the increase in

fatigue life depends upon the relative stiffnesses of the bolt and the

assembly, which includes the bolted materials, the nut, and all other

parts which are elastically deformed. It is desirable that the bolt have

*See, for example, the derivations in references 8 or 12
- 4 -

as low a stiffness and the assembly as high a stiffness as possible.

This fact should be borne in mind in the discussions of individual

design features which follow.

Two practical factors limit the attainment of maximum benefits from

preload. One is general ignorance of the exact magnitude of the preload

being applied. The second is the gradual loss of preload which is experienced during use, resulting from loosening of the nut, wear of mating

surfaces, stress relaxation, etc.

It has been estimated that only about a tenth of the torque applied

to a bolt goes for preloading, the remainder being required to overcome

friction. At least one manufacturer (ref . 16) states that when friction

conditions are "noirmal" the applied torque should equal about twenty

percent of the product of the bolt diameter and the desired preload.

Average deviation has been quoted as seven percent, but deviations of the

order of twenty-five percent are apparently not uncommon with aircraft

bolts

In special field applications gaged bolts, such as the proprietary

Pic, Stralnsert and Tru-Load bolts, can be used to give indications of

preload. A more common but less versatile device for this purpose is

the so-called preload indicating washer.

Periodic retlghtening of the bolt is required to restore preloads

which have diminished during use. The rate of loss of preload varies

Inversely with the magnitude of the preload, and, for a given preload,

the rate of loss varies inversely with the torque required. Sealing

compounds, fabrics, rubber, or a heavy build-up of paint on the faying

surfaces are undesirable since these extrude and leave a loose joint.

Sometimes, however, they are necessary to minimize fretting, as will be

shown later. The use of spring washers to maintain preload is a controversial practice since they also reduce the stiffness of the bolted

assembly.

Preloaded bolts are preferable even when the bolt is subjected to

working loads in shear. The clamping forces resulting from preloaded

bolts reduce slip in the joint, and greater portions of the working loads are then carried by plate friction. This reduces bending of the bolts

which is severely detrimental to fatigue life, particularly when the

bending energy must be absorbed In the threaded portion of the bolt.

With sufficient friction, the bolt will not feel small load fluctuations

which would otherwise cause fatigue damage. Preload benefits of this

type are greatest with thin sheet joints where the amount of load transferred by friction can be made large compared to that transferred by

the bolts.
- 5

-

The eventual fatigue fracture of a joint which carried most of its

load by friction is, perhaps, difficult to visualize. For this, the

reader is referred to the excellent photograph in Fig. 2c on p. 1355

of ref. 9.

Special wrenching recesses, such as the patented Torq-Set and

Hi-Torque recesses, have been designed to permit the application of high

preloads to shear bolts which do not usually have very large heads. See,

for example, bolts with flush countersunk heads, such as the NAS 1151

and 1220 series.

Several exceptions to these general rules should be pointed out.

When the external fatigue loads are of an impact nature rather than

smoothly cycled, high values of preload act to reduce the fatigue life

of the bolt (ref. 14). Under certain conditions it is desirable to

forego the benefits of preload in order to minimize the possibility of

stress corrosion. In materials in which fatigue life is relatively

sensitive to mean stress, preloading may be of questionable value.

Finally, the designer is cautioned against using bolt preload as a design parameter unless a foolproof technique for maintaining the desired

amount of preload can be devised. Rather, bolt preload should be looked

upon as a bonus value; a means for achieving reliability through fatigue

lives which are greater than those actually specified.
- 6 -

3. FATIGUE OF BOLTS

Virtually all fatigue fractures of bolts occur in one of three

places:

(1) At the first thread, that is, at the intersection of

the thread with the shank

(2) At the first working thread, which is the first

thread engaged by the nut

(3) At the shoulder, where the shank joins the head.

Increasing the fatigue strengths at these critical locations involves

the use of proper thread shapes, materials, fabrication processes, nuts,

fillets and other devices which minimize stress concentrations and notch

sensitivities. These subjects are covered in the subsections which

follow.

3.1 Shape of Bolt Threads

(Supplementary references: 17 through 24)

a. Root radius

This is the most important of the geometric considerations which

affect the fatigue strength of threads. Fatigue strength increases with

root radius. A V-bottomed thread gives notoriously poor fatigue

strengths and the flat-bottomed American National thread is not much

better. The Unified thread (MIL-S-7742A) , which is the basis for most

present-day aircraft bolt threads, permits the thread roots to be

rounded, either intentionally by the use of a rounded tool or inadvertently as a result of tool wear. MIL-B-7838A improves the situation by

actually specifying minimum acceptable root radii. Specification

MIL-S-8879 goes a step further by requiring even greater root radii

which reduce the thread depth to only 75 percent of the basic value

instead of the 83 percent allowed by MIL-S-7742A. Presently in the

developmental stage is a still newer modification which provides for

still greater root radii that reduce the thread depth to only 55 percent

of the basic value. This feature is particularly desirable for extremely

notch sensitive materials such as beryllium .

b. Flank angle

The included flank angle for most standard thread systems is either

55 or 60*. Tests have shown that varying this angle from 45 to 65* does
- 7

-

not affect fatigue strength significantly. However, increasing the

angle to 90* has been shown to prolong fatigue life significantly.

This feature is not now employed in aircraft bolts.

c. Depth of Engagement

In the Unified thread system, the depth of engagement of mating

thread surfaces is nominally about 88 percent of the bolt thread depth.

Within limits , truncation of the bolt thread crests does not affect its

fatigue strength. However, a combined truncation of both the bolt and

the nut threads , giving a depth of engagement of only 25 percent of the

normal value, increases the fatigue strength by some 40 percent. The

improvement is attributed to the better distribution of the load among

the nut threads. This is most effective when the nut material is

softer than the bolt material and when the bolt thread crests are

rounded off following truncation.

d. Pitch

Cursory investigations have often led to the conclusion that

decreasing the pitch of a thread raises its fatigue strength. However,

it is now fairly well established that the beneficial effect resulted

from the fact that smaller pitches are generally accompanied by larger

minimum diameters. If the minimum diameter is held constant, fatigue

strength is actually reduced slightly by decreasing the pitch.

e. Taper

Tapering bolt threads in a direction similar to pipe threads

reduces fatigue strength. Tapering in the reverse direction, although

generally impractical except in the case of specific studs, increases

fatigue strength when used with untapered nuts.

f. Lok-threds

An entirely different, and less common, approach to thread design

for fatigue strength is achieved by the use of threads which, in effect,

create force fits. An example of this is the patented Lok-Thred. This

thread is flat-bottomed, like the American National, but the minimum

diameter is larger, the flat bottom is wider, and it is tapered at 6* to

the axial direction. In cap screw or stud applications or with nuts

softer than the bolt material, the Lok-Thred is reputed to reform the

female thread, cold work it, and induce compressive stresses in it. This

locking fit, which is contended to be replaceable, is said to eliminate

cyclic wear on the mating surfaces and to counteract the tendency of the

preload to lessen. The result, according to the manufacturer, is a high

fatigue life for both the bolt and the female part. Available test data

are inadequate to verify these claims
- 8

-

3.2 Thread Forming

(Supplementary references: 19, 20, 25 through 32)

Two processing factors which are most important for high fatigue

strength in threads are smooth surface finish and the development of

surface ccaapressive stresses. The best way to obtain both of these

conditions is by roll- forming the threads after the bolt blanks have

been heat treated. Threads formed by rolling have far greater fatigue

strengths than machined threads, whether they be turned, milled or ground. Thread rolling tends to produce a maximum thread diameter

which is greater than the diameter of the original bolt blank. However,

many aircraft bolt specifications forbid thread diameters which exceed

the shank diameter. Hence, the length of the blank which is to be

threaded is frequently reduced in diameter prior to thread forming.

If gross decarburization develops during the heat treating process,

the decarburized layer must be ground off prior to thread forming. If

the heat treating process involves a quench hardening, a tempering

treatment is desirable to relieve the quenching stresses. With carbon

steel bolts, it is often more desirable to substitute work hardening for

the heat treatment prior to thread forming. Heat treating after thread

rolling should be avoided since this relieves the beneficial compressive

residual stresses.

It is interesting to note that thread rolling cold works the sur- face layer in addition to creating compressive stresses. In a sense,

this is undesirable since it raises the effective stress concentration

factor. The net effect, however, is decidedly beneficial. In some materials the rolling process produces fine cracks in the surface. If

the compressed layer is thicker than the crack depth, and if the material

has a low rate of crack propagation, then the fatigue resistance is

nevertheless improved. This, however, limits the degree of rolling that

is beneficial for a given material.

Most aircraft bolts are manufactured with rolled threads despite

the fact that most bolt procurement specifications do not require this.

The reason is that rolling is a rapid means of thread forming and also

an economical one when the volume of production is high enough to warrant

the special equipment and tools needed. In the event that a small quantity of special bolts is required, the specification of rolled threads

can be omitted unless fatigue failure in the bolt threads is anticipated.

Shear bolts for specific pure shear applications, for example^ do not

necessarily require rolled threads since, with the proper grip length,

failure will occur in the unthreaded shank. Although rolling increases

the fatigue strength of threads, it does not increase the static strength,

and hence thread rolling does not influence the preload capacity.
- 9

-

Other methods for producing residual compressive surface stresses

are available when roll-forming is impractical. One method, of course,

is to roll the thread roots alone after the machining process. Other

methods are carburizing, nitriding and cyaniding. Special care must

be taken with carburizing since, under certain circumstances, the

residual surface stresses achieved with this technique may be tensile,

rather than compressive. Also, the quenching process which follows

carburizing may form cracks. Nitriding, on the other hand, virtually

always produces a compressed crack- free surface. The disadvantage of

case-hardening methods, regardless of the depth of the case, is that

they increase the brittleness of the surface of the part.

Irregularities in thread surfaces are eventually smoothed out by

alternating loads and result in a slackening of the preload. For this

reason, smooth thread surface finishes are desirable for maintaining

preload. Most aircraft bolt specifications which call for rolled

threads, such as MIL-B-7838A, require that the surface roughnesses of

the thread flanks and roots not exceed 32 microinches (measured according to MIL-STD-lOA). Certain other aircraft bolt specifications, such

as NAS 583, also cite this roughness value, and therefore implicitly

require that the threads be rolled, since lathe-cut threads commonly

yield roughnesses of the order of several hundred microinches. While

grinding can also produce very smooth thread surfaces, careful techniques are required to avoid introducing residual tensile stresses at

the same time.

Regardless of the thread forming process used, careful inspection

is necessary to detect flaws. A systematic program of tests conducted

in a direct stress fatigue machine showed that it is important to detect

longitudinal flaws as well as transverse flaws. Longitudinal seams,

laps and inclusions in aircraft bolts reduce fatigue life as much as thirty percent.

The fatigue strengths of stress-relieving grooves (see Section 3.3) are also improved by rolling. Alternatively, shot peening can be used

to develop compressive stresses in the groove surfaces.

3.3 Improving Fatigue Strength at the First Thread

(Supplementary references: 31 and 33)

In terms of fatigue strength, a continuous thread is superior to a series of circumferential grooves having the same pitch. Furthermore,

the series of grooves is superior to a single circumferential groove

because the interference of adjacent stress concentrations results in a cancelling effect.
-loin spite of this effect, threads are notoriously fatigue sensitive.

In one series of investigations it was found that the shank diameter

of bolts could be reduced to three-fourths of the root diameter of the

threads (57 percent of the area) and fatigue failures still took place

in the threads. In fact, the result was an improvement in fatigue

strength for reasons which are discussed later.

The first thread on a bolt does not benefit from the presence of

interfering stress concentrations on both sides of it. It is, therefore, one of the three most common sources of bolt fatigue failures.

Special efforts must be made to reduce the stress concentration at the

first thread, and it is usually imperative to keep this thread away

from the nut, which introduces stress concentrations of its own.

A common practice for reducing stress concentrations at the first

thread is to run the thread out gradually. This process generally raises

fatigue strength to satisfactory levels but may not be sufficient if the

bolt is subjected to bending. When bending of the bolt is expected, it

is often desirable to undercut the shank with a smooth groove just beyond

the threads to a depth slightly greater than the thread root. The smooth

groove has an inherently lower stress concentration factor than a thread

and exerts a mitigating stress interference effect on the first thread.

Furthermore, bending energy is now absorbed over the length of the undercut, whereas it is otherwise primarily absorbed in the threads. Some

investigators report that in the presence of a stress-relieving groove

it is permissible to engage the first bolt thread with the nut so long

as this thread is kept away from the seating face of the nut. This is

done by torquing the nut beyond the first thread so that it projects over

the groove.

When severe bending is anticipated, a number of undercuts should be

employed to provide flexibility. Sufficient spacing should be allowed

between the undercuts to permit the bolt to bear on the bolt hole surface

properly, and to provide a full cross section at the faying surface.

This type of bolt thus becomes a specialized part, suitable only for its

particular application. See, for example, Fig. 1.

A procedure which has been suggested as an alternative to undercutting involves drilling down the center of the shank from the head to

a point just before the first thread.

Most standard aircraft bolts do not employ stress-relieving grooves

or drilled shanks, but rely rather on gradual thread runout. The only

notable exceptions are the NAS 563 series and shoulder bolts such as NAS 129 7. The slightly reduced shank diameters available on some engine

bolts such as the MS 9033 series can hardly qualify as undercuts. Nevertheless, fatigue failures of aircraft bolts at the first thread have

become a rarity; this attests to the suitability of the thread runout

techniques.
- 11 -

3.4 Effect of Nut Design

(Supplementary references: 14, 17, 21, 31, 34 through 40)

The fatigue strength of engaged threads in tension is only about

two- thirds of that of unengaged threads. This situation, is due, in part,

to the fact that engaged threads do not share the load uniformly. In a bolt-nut combination, the first engaged thread carries the greatest

share of the load, and following threads carry smaller and smaller

shares. The result is that most fatigue failures of bolts occur in the

first engaged thread.

It is obvious that a nut design which will provide a more uniform

distribution of load over the working bolt threads will generally in- crease the fatigue strength of the bolt. Hence, while a thorough discussion of nuts is beyond the scope of this report, the subject must be

included to some extent because of its importance in bolt fatigue.

Many nuts designed to distribute bolt load more evenly have been

produced and many more have been proposed. Most of these involve

reducing the effective stiffness of the nut relative to the bolt. It

will be recalled that, to obtain maximum benefits from preload, the nut

should be stiff. However, the improved load distribution obtained with

a less stiff nut generally outweighs the reduction in preload benefits

which accompany it. Some alternative principles of nut design and

selection which have been set forth to improve load distribution among

threads are listed below. These may be used individually or in combinat ions

(1) Use nuts having a lower elastic modulus than the bolt.

(2) Use nuts having a lower yield strength than the bolt.

(3) Undercut the working face of the nut (see Fig. 2a).

(4) Taper the nut threads

(5) Use a slightly greater pitch for the nut threads than

for the bolt threads.

(6) Use a tapered lip on the nut (see Fig. 2b).

(7) Use a concave or convex working face on the nut with

a suitably matched washer (see Fig. 2c).

A commercially available nut having a so-called "Equa-stress thread

form" incorporates items (4) and (5), above, and reportedly increases

aircraft bolt fatigue life by more than one hundred percent.
- 12 -

Another patented product employs a collar around part of the nut to

exert a radial pressure. According to the manufacturer, this diffuses

the stresses in the nut and the bolt more uniformly and permits the

application of greater preloads without exceeding allowable stresses in

the threads

The addition of a second or even a third nut to a bolt-nut combination is desirable. Tightening the second nut against the principal nut

reduces the stress at the first working thread. The fatigue benefits so derived increase with the internut torque applied.

The thread length of the nut should be at least half the bolt

thread diameter to prevent stripping of the threads. Increasing the

thread length beyond this point may increase the static strength of the

bolt, but only adds to its fatigue strength in that its preload capacity

may have been raised. Available evidence on the question of how far

the bolt should protrude beyond the nut is contradictory.

Under- tightening of aircraft bolts frequently occurs because a cas- tellated nut is adjusted simply to enable the split-pin to be inserted.

This error would be much reduced if two suitably oriented holes were

drilled in the bolt end instead of one.

It was formerly believed that forcing materials, such as nylon

pellets or hemp packing, into threads increased fatigue life. Recent

evidence, however, shows that non^metallic locking elements generally

disturb the load distribution and reduce fatigue life. The sole

advantage of a non-metallic locking element is that it resists complete

loss of the nut by vibrations when the preload has completely disappeared,

In spite of all the work that has gone into nut development, most

tensile fatigue failures of aircraft bolts Occur in the engaged threads.

It is clear that further increases in the tensile fatigue properties of

bolts will have to come from advances in materials, thread shapes, thread

processing techniques and/or nut design.

3.5 Bolt Head and Shoulder Design

(Supplementary references: 21, 29, 41 through 44)

Fatigue failures of aircraft bolts occasionally occur at the juncture between the bolt head and the shank. This danger is minimized by

using as large a fillet radius as possible, and by properly machining

the underside of the bolt head so that preloading is truly axial.

Where a large fillet radius cannot be used, several alternatives are available. One is the use of a reentrant fillet. This has the added
- 13 -

advantage of providing a more flexible bolt for greater preload benefits

and also reduces offset loading tendencies. A second alternative Is the

use of an elliptical fillet vhlch Is sometimes superior to a circular

fillet of equivalent size. As a last resort, the shoulder can be flame

hardened; this results In a stress distribution similar to that obtained

with a fillet.

For greatest fatigue resistance, the bolt head should be forged to

dimensions closely approaching the final dimensions In order that the

favorable allnement of the metal fibers may be retained after machining.

Also, the fillet should be cold rolled. The following data compare

fatigue lives for several bolt head manufacturing processes.

Process Average cycles to failure

Hot forged, with rolled fillet 100,000

Machined from bar stock 25,000

Cold upset with machined socket 20,000

Hot forged complete 17,000

The fatigue strengths of conical head bolts generally are lower

than those of bolts with conventional heads, particularly \^en deep

wrenching recesses, such as Phillips or Reed and Prince recesses, are Involved. The head-to-shank juncture is notably fatigue sensitive, and

careful machining of both the head and the countersunk hole is required

to Insure that the cone axis is alined. This minimizes bending stresses

in flexible joints and provides proper bearing surface.

A recent commercial development is the self-alining nut ^Ich is a ball-and-socket type of nut -and-washer arrangement. The combination is

reported to correct offset loading tendencies up to eight degrees when

bolting non-parallel surfaces.

When high strength bolts are used on soft materials, It is desirable

to enploy hardened steel washers to permit greater preloading without

brinelllng. Thick washers should also be used under bolt heads and nuts

when the holes are oversized, in order to better distribute the clamping

forces . Experimental evidence shows that hardened steel washers do not

reduce fatigue strength. Nevertheless, in the Interest of simplifying

assemblies, at least one aircraft bolt manufacturer now produces a line

having extra-wide heads so that washers are not required.
- 14 -

3.6 Bolt Materials

(Supplementary references: 24, 28, 45 through 47)

The desirable properties for bolt materials are: Low notch sensitivity, medium damping capacity, high stress relaxation resistance, high

endurance limits, and high tensile strength. Examples of classes of

materials which meet most or all of these requirements are: Low carbon

steels, nickel steels, Cr-Ni and Cr-Ni-Mo steels, nitrided steels, and

2017 and 2024 aluminum alloys.

Another important consideration is that notch sensitivity generally

increase's with material hardness. Hence, heat treating to higher

strength levels is not beneficial if the notch sensitivity is increased

as well. As recently as 1946, it was believed by some that no fatigue

advantage could be gained by using steels with strengths in excess of

140,000 lb/ in? Today, however, by maintaining a proper ratio between

the yield and tensile strengths, and by using thread configurations which

minimize the stress concentrations at the thread roots, manufacturers are producing fatigue resistant bolts having strengths up to 300,000 lb/ in?

Most aircraft bolts continue to be made of steel although in recent

years greater numbers of non-ferrous bolts have been produced. Low alloy

steels such as AISI 4340 and 8730 predominate with stainless steel types

302, 347, 431 and others being used for corrosion resistance.

Titanium alloy bolts often show greater fatigue resistances than

comparable steel fasteners even at temperatures up to 500° F. Some of

the more highly regarded titanium alloys for this purpose are 7Al-4Mo,

6A1-4V and 13V-llCr-3Al. Additional advantages of titanium alloy bolts

are that; they do not rust and that they weigh one-third less than steel.

Indeed, if titanium alloy bolts were substituted for all of the steel

bolts in the Northrop F-89, the overall weight would be reduced by about

100 lb while, on an eight -engined heavy jet bomber, the saving would be

more than 1200 lb (ref. 45). The deterrent to large scale use of

titanium alloy fasteners is their high cost which results not only from

the cost of the material but also from the fact that manufacturing

techniques must be tailored to each individual lot of material in order

to achieve a uniform product. The value of weight-saving must be high

to justify wholesale use of titanium alloy bolts.

The same economic considerations apply, only more so, in the case

of beryllium bolts. Vfliile not yet available except in limited quantities,

beryllium bolts have demonstrated a superiority to both steel and titanium

bolts in terms of static and fatigue strength-to-weight ratios. In com- parison with stainless steel bolts, beryllium bolts maintain a superiority

up to 850° F. The atmospheric corrosion resistance of beryllium is

similar to that of aluminum.
- 15 -

Further information on bolt materials is presented in Section 5.1.

3.7 Platings

(Supplementary references: 29, 31, 48 through 52)

Plating of aircraft bolts is required to resist oxidation and

corrosion, and also to act as a lubricant to resist galling. The important considerations are to avoid platings which deposit with residual

tensile stresses and plating processes which induce embrittlement.

Electrodeposits of cadmium, tin, lead, zinc, copper or silver will

improve the corrosion- fatigue resistance of steel bolt threads without

seriously impairing the normal fatigue properties, providing the necessary precautions are taken during the plating process to minimize

hydrogen embrittlement.

Most aircraft bolts for normal environments are cadmixim plated in

accordance with QQ-P-416. The susceptibility of this process to hydrogen

embrittlement increases with the thickness of the plate and the hardness

level of the bolt. While dense platings provide greater corrosion

resistance, they also prevent hydrogen from being effectively outgassed.

Hence, the thickness of cadmium platings on aircraft bolts is generally

kept below 0.0003 inch. With steel bolts heat treated to hardnesses

above about Rockwell C46, vacuum deposition of the cadmium per MIL-C-8837

is recommended to avoid embrittlement.

The effectiveness of cadmium plating as a lubricant is controversial. One source reports a fifty percent reduction in driving torque as

a result of the plating, while another insists that it actually creates

galling. In any event, it is generally agreed that all bolts which will

require periodic removal or retightening should be given a supplementary

oil or grease lubrication. Molybdenmn disulfide and graphite dry film

coatings in suitable binders are occasionally added for increased lubrication and prevention of galling.

In certain types of moving- structure joints, such as wing folds,

aircraft bolts serve as shafts. Cadmium platings and dry film lubricant

coatings are frequently bypassed for these applications since they tend

to deteriorate under the fatigue action of oscillatory motions accom- panied by high bearing loads. Chromium platings of bolt shanks are more
- 16 -

durable, and are often specified for these applications because of their

low friction coefficients. Commercial and industrial hard chromium

platings should be avoided since these are normally deposited in a relatively crack-free state of residual tension. Special chromium plating techniques have been developed in recent years which leave a network

of many fine hairline cracks in the plating. These do not act as fatigue

nuclei but rather prolong fatigue life by relieving the otherwise-present

residual tensile stresses. They also provide reservoirs for the lubricant,

A certain amount of hydrogen embrittlement is normally associated with

chromium plating, but this has not been found to be excessively

deleterious. Baking at temperatures to about 400* F to eliminate it

generally results in a lowering of the fatigue strength. Baking at

750* F is desirable, however. The thermal expansion of chromium is

about half that of steel, and the high temperature bake stretches the

chromium beyond its elastic limit, thereby providing further stress

relief. Residual compressive stresses in chromixim platings have even been achieved in this way.

Two methods are available for obtaining satisfactory chromium platings using ordinary plating processes . The plating can be etched to

provide stress relieving cracks, or the bolt shank can be peened prior

to plating per QQ-C-320. Apparently, the peening raises the fatigue

strength and the plating reduces it. The net effect has been reported

as an increase in fatigue life of seven to ten percent over unpeened,

unplated specimens

Even with the most up-to-date techniques, however, the fatigue

strengths of chromium plated bolt shanks are limited by the fatigue

strength of the chromium. Failure of the plating creates a notch effect

which leads to rapid failure of the steel shank. There is little advantage, therefore, in using very high strength steels in these applications . Incomplete bonding of the chromium plating should also be avoided

since this, too, acts as a crack nucleus.

Bright nickel platings behave similarly to chromium platings in most

respects but are not widely used on bolt shanks for moving- structure

joints because of their relatively poor fatigue and friction properties

Dull nickel platings generally embody residual tensile stresses.

Further information on platings is given in Section 5.3.

3.8 Civil Engineering Bolts

The so-called "high tensile" or "high strength" steel bolts used

in civil engineering applications should not be confused with aircraft

quality fasteners. These civil engineering bolts are made of medium

carbon steel with tensile strengths of 90,000 to 120,000 lb/in. 2 and
- 17 -

yield strengths of 77,000 to 88,000 lb/ in? The threads may be either

cut or rolled.

In civil engineering applications, these bolts require hardened

steel washers under the head and the nut to prevent brinelling. Most

studies show that when properly installed, they give higher fatigue

strengths than similar riveted joints.

For further information see ASTM Designation A325-58T and the

collection of articles on. the subject in Fasteners . Vol. 15, Nos. 2 and

3, Fall-Winter 1960.

3.9 Cap Screws

(Supplementary references: 12, 29, 35)

When a tensile force is applied to a cap screw which is threaded

into a tapped hole in a structural member the stress pattern in the

structural member is much more favorable than the pattern which develops

in a nut (see Fig. 3). The distribution of load among the threads is

more uniform with the cap screw than with the bolt -nut combination. For

this reason greater fatigue lives are normally attained with properly

used cap screws than with comparable bolts. When the cap screws are used in light alloys which have lower elastic moduli, the fatigue

strengths of the screws approach twice those of bolts.

As with a bolt, cap screws should be designed so that the first

working thread is not located at the face of the seating. Also, the

shank should be undercut or a stress relieving groove provided when

bending is present. When these precautions are taken, fatigue failure

frequently occurs at the thread closest to the bottcwn of the tapped

hole. The fatigue strength at this location may be improved by axial

drilling of the threaded end of the screw to decrease its stiffness and

further distribute the load.

Cap screws should not be permitted to touch bottom in the tapped

hole because no preload can then be developed.

4. FATIGUE OF BOLTED JOINTS

The effective stress concentration factor for a bolted joint may

vary from about 3 to as high as 13 depending upon the detail design.

This is a tremendously important consideration affecting the fatigue

life of a joint. Relatively simple changes in detail design may improve

life by a factor of 10 to as much as 100. As general rules, wherever

possible, eccentric loading should be eliminated in order to minimize

tension and bending on shear bolts, and symmetrical designs should be

utilized.
- 18 -

Experience has shown that, for a joint material with a given ultimate

strength, better fatigue properties are obtained with lower values of

yield strength. Preliminary results (ref. 53) suggest that the ratio of

yield strength to ultimate strength should not exceed about 85 percent.

4.1 Joint Configuration

(Supplementary references: 11, 13, 54 through 57)

a. Single shear Joints

In aircraft structures, the most fatigue-sensitive parts are generally those single shear (lap) joints in which rivets or bolts must resist

a load, as distinct from those where the rivets or bolts merely hold an unbuckled skin on the frame. For conventional spacings, the stress

concentration in the former case is usually about 5 1/2, while in the

latter case it approaches 3.

Single shear joints are poor in fatigue because of their inherent

eccentricity which leads to bending of the bolts. Under this condition,

the bearing loads are restricted to a small area of the hole wall and

this results in increased stress concentrations at the holes.

Bending can be minimized by exercising extreme care in the design

of the splice (or doubler) plate. Obviously, bending is reduced if the

doubler is made as thick as possible. This, however, exaggerates the

unequal distribution of load among the bolts which is a function of the

difference in stretch of the upper and lower sheets between the first

and successive rows of bolts. This unequal distribution can be reduced,

in turn, by spacing the bolts closer together and by proper use of bolts

of different moduli. Further improvements are possible by using a tapered doubler, by using a stepped laminated doubler, or by using an auxiliary thin doubler to take most of the bending at the first bolt.

Bonding the splice plate to the main plate is also helpful in that

bending then starts at the edge of the splice rather than at the first

row of bolts. Still another device involves the addition of a bolt just

outside of the splice so that the first loaded bolt benefits from the

mitigating stress interference effects of bolts on both sides of it.

b. Doubly shear joints

Double shear (or double strap) joints have the advantage of involving no eccentricity and bending. Further improvements in the fatigue

strengths of these joints can be obtained by using a transverse stress

relieving groove (Fig. 4a) or reduced plate thickness (Fig. 4b). With

these modifications no special care need be taken with hole preparation

(see Section 4.5) since, if the bolts are properly prestressed, fatigue

failure generally takes place at the edge of the groove or fillet. As
- 19 -

vith lap joints, however, it is desirable to use a high concentration of

bolts in a small area to minimize xinequal bolt loading.

I

c. Scarf joints

Little experimental evidence is available in the literature on the

fatigue properties of single scarf joints. The quality of this configuration is apparently very sensitive to detail design, material, and

assembly procedure.

Double scarf joints, on the other hand, are the strongest joint con- figurations in fatigue because they involve no eccentricity and because

they permit the bolts to be preloaded in shear.

4.2 Joint Proportions

(Supplementary references: 10, 11, 17, 18, 20, 21, 41, 58)

Having chosen a joint configuration, the next step in the design

problem (aside from materials selection) is to select a bolt diameter,

sheet thicknesses, and the number of bolts to be used. Insofar as fatigue strength is concerned, the criteria which govetti these selections are as follows:

(1) Per unit of cross sectional area, smaller bolts are

stronger in fatigue than larger ones. This is primarily a matter of stress concentrations rather than

a metallurgical size effect, and the differences are

small for near or adjacent sizes.

(2) Larger bolt diameters permit larger preloads and

greater clamping forces . This leads to greater

proportions of the joint load being transferred by

plate friction and less fretting of the joint.

(3) Per unit of joint thickness, thinner joints are stronger in fatigue than thicker ones because, in

thinner joints, greater proportions of the total

load are transferred by plate friction.

(4) Higher ratios of bolt diameter to sheet thickness

result in less bending of the bolts. Bolt bending

reduces the fatigue strength of the sheet as well

as of the bolt by creating a non-uniform distribution of bearing stress through the sheet thickness.

(5) When a considerable portion of the bolt load is in

tension, the grip should be as large as possible to

minimize bending stresses resulting from possible

nonaxial bolt loading.
- 20 -

(6) Increasing the number of bolts in a joint permits

the clamping force to be distributed more uniformly

over the entire joint area. This reduces both the

bearing stresses and the tendency to slip.

Of these six considerations, the first one is generally of secondary

importance and may usually be disregarded. The second, third and fourth

considerations are compatible with one another. The problem is thus

ordinarily reduced to employing as high a ratio of bolt diameter to sheet

thickness as possible, and as many bolts as possible. Space limitations,

however, would generally require that bolt diameter be reduced if space

were needed for more bolts, or that the number of bolts be reduced if

space were needed for larger ones. Achieving the most favorable balance

between number and size of bolts, then, is often a test of the designer's

experience

It should be noted, however, that if the tensile fatigue strength

of the bolts is the critical factor, the bolt diameter should never be

made greater than the grip. In certain cases it is even desirable to

increase the grip by inserting washers or collars.

4.3 Bolt Pattern

(Supplementary references: 11, 41, 55, 57)

Adding more bolts to a joint raises its fatigue life because the

increased clamping reduces slippage, and because the bearing stresses

are reduced.

For a bolt row which is transverse to the direction of load,

widening the joint and adding proportionately more bolts increases the

fatigue strength of the joint approximately proportionally. If the

bolts are spaced closer together, the ratio of fatigue strength to

static strength increases. The optimum spacing is about 2 1/2 diameters.

For a bolt line which is parallel to the direction of load, adding

more bolts raises the fatigue strength of the joint but lowers the

average fatigue strength per bolt because the load is not uniformly

distributed among the bolts. The first bolt carries the greatest share

of the load. If the bolts are spaced closer together, the load distribution becomes more uniform and the fatigue strength is increased.

Another method of reducing the peak loads taken by the first bolts

involves the use of aluminum bolts on the outside and steel bolts on the

inside. Still another technique which has been suggested for achieving

the same effect calls for smaller bolts on the outside and larger ones on the inside. Tests have demonstrated the latter suggestion to be

invalid, however. In any case, it is undesirable to employ a mixture

of fasteners in a joint, such as rivets and bolts, or tension bolts and

shear bolts.
- 21 -

As a general rule, best results are obtained when each row contains

the same number of bolts, and the different rows are In line with one another

4.4 Preloading of Joints

(Supplementary references: 58 through 62)

In Section 2, it was shown how the preloading of bolts increases

the fatigue life of both the bolts and bolted joints. This section will

discuss further improvements in the fatigue life of joints which can be

attained by preloading the joints themselves. In a sense, the use of the

term "preloading" for both processes is unfortunate, since the two processes are quite different. When referring to bolts, the preload is a state

of tension in the bolt which is created by torquing the bolt; every effort

should be made to prevent the tension from diminishing during the subsequent fatigue loading. In the case of joints subject to tensile fatigue,

the preload is a tensile load which is applied to the joint and removed

immediately thereafter, prior to the fatigue loading.

The application of a high tensile preload to a joint creates local

stresses in the vicinity of the bolt holes which exceed the yield strength

of the material. When the preload is removed, local peirmanent tensile

deformations remain, and a state of residual compressive stress is created

in the vicinity of the holes. These residual compressive stresses reduce

the effective mean stress which is actually "felt" by the material in the

vicinity of the holes during the ensuing tensile fatigue loading. The

fatigue properties of most structural aluminum alloys are quite sensitive

to mean stress so that substantial increases in the fatigue life of joints

fabricated frcwa these materials are obtained when the effective mean stress

is reduced by preloading. The fatigue properties of some steels, on the

other hand, are relatively insensitive to mean stress. Joints fabricated

from these materials are, therefore, relatively unaffected by this kind of

preloading.

Note that this effect is, to a certain extent, opposite to that produced by the preloading of bolts where the mean stresses are raised, albeit

with a simultaneous reduction in the alternating stress amplitude.

It is worth mentioning that preloading cannot significantly alter the

fatigue properties of an ideally homogeneous specimen which does not have

any stress concentrations since no macroscopic residual stresses can be

created. This is one reason why such poor correlation is frequently en- countered between the fatigue properties of materials and those of structures.

Preloading also increases the fatigue life of joints in another way.

The plastic flow which takes place in the vicinity of the holes permits

the subsequent fatigue loads to be more uniformly distributed among the
- 22 -

various bolts. It Is this effect that produces somewhat greater preload

benefits with joints than with other tjrpes of structural stress concentrations.

Obviously, no benefits can be obtained unless the preload Is high

enough to create some localized plastic deformation. Raising the preload

raises the fatigue life until some optimum amount of deformation Is

reached. Preloads greater than this apparently Introduce excessive hole

elongation or other damage. The actual magnitude of the life Increase

obtained through preloading depends on the nature of the subsequent

fatigue loading as well as on the magnitude of the preload and the joint

design. It has been conjectured that the residual compressive stresses

which are created by tensile preloading tend to fade away during the

ensuing fatigue loading, particularly when the fatigue stresses are high

with respect to the yield strength. This would explain why the relative

Increase In fatigue life for a given preload decreases as the maximum

load of the fatigue cycle Is Increased. This would also explain why

spectacular Improvements In fatigue life are obtained when the residual

stresses are periodically restored by the application of single overloads

at long Intervals during the fatigue process, even when the overloads are of only moderate Intensity. On the other hand, the application of a series of preloads prior to the Initiation of fatigue loading results In

fatigue life benefits which are only sightly greater than those obtained

with a single preload.

Thus far, this discussion has been concerned with tensile preloads

which are followed by tensile fatigue. The case of compressive preloads

followed by compressive fatigue Is qualitatively similar. Tests have

substantiated the Intuitive conclusion that preloads which are applied

In the direction opposite to the subsequent fatigue loading create

residual stresses which raise the effective mean fatigue stresses and

thereby reddce fatigue llf€^.

The proper choice of a joint material Is all-important if there is a desire to employ the benefits of preloading. Unfortunately, this choice

is not easy to make with the limited fatigue data available for various

materials at various mean stress levels. It has been found, for example,

that preload benefits are greater with joints fabricated from an extruded

aluminum alloy than with identical joints fabricated from sheets of the

same material. The reasons for this are not yet clear.

While structural preloading appears to be a powerful tool, it cannot

be accepted as a design parameter as yet because it is not fully understood nor have all its implications been fully explored. Like bolt preloading, the best use for structural preloading may be as a means for

achieving reliability through the attainment of fatigue lives beyond the

design requirements.

One interesting corollary of the preload phenomenon should be emphasized. It Is a questionable practice to evaluate the fatigue life of a
- 23 -

structure using a specimen which had been previously tested to static

failure and then repaired. This procedure may give an unrealistically

high fatigue life and, perhaps, fatigue failure in the wrong place.

4.5 Hole Preparation

(Supplementary references: 11, 20, 35, 58, 63 through 70)

The fatigue strength of a strip with a hole is about 10 percent less

for a threaded hole than for a smooth hole. If the hole is loaded with

a bolt, the difference becomes much smaller.

The remarks made earlier regarding plating of bolts apply equally

well to the plating of holes in thick lugs. Proper cadmium plating of

holes was found to cause no serious reduction in fatigue properties , but

chromium plating caused an eight-fold reduction in fatigue life, presumably

due to the residual tensile stresses which frequently accompany this process

An interference fit between a bolt and the unthreaded hole boundary

increases fatigue life by reducing fretting and impacting between the bolt

and the hole boundary and by changing the stress conditions at the

boundary. Under externally applied fatigue loading, an interference fit

effects a rise in the mean stress level at critical points on the hole

boundary, together with a simultaneous fall in the alternating stress

amplitude. This phenomenon is entirely analogous to the beneficial changes

in stress conditions which are created in bolts by preloading.

The beneficial effects of interference fits do not become significant until the ratio of the bolt diameter to the hole diameter exceeds a certain threshold value. This value depends, to some extent, on the

materials involved, but is generally about 1.003. (Small changes in the

modulus of elasticity of the bolts affect the fatigue properties only

slightly as compared to the effects of very minute changes in the degree

of interference.)

As the bolt-to-hole diameter ratio is increased, the mean stress

level also increases, but the alternating stress amplitude decreases

only until a certain limiting value of interference is reached. Beyond

that point, the alternating stress amplitude remains essentially un- changed. The magnitude of this limiting value of interference depends

on the ratio of plate width to hole diameter. The most desirable degree

of interference, however, depends on the relative influences of mean

stress and alternating stress on the fatigue resistance of the plate

material involved and may be less than this limiting value.

In any case, of course, the degree of interference employed should

not be so great as to hamper the proper preloading of the bolts , nor to

create plate stresses exceeding the yield strength. For materials subject
- 24 -

to stress corrosion cracking or hydrogen embrittlement, the degree of

interference must be reduced further. BuWeps specification SD-24J re- quires, in this case, that the interference stresses at the hole boundary

not exceed 50 percent of the material's yield strength in the longitudinal grain flow direction, 35 percent in the long transverse and 25 percent in the short transverse direction.

An alternative means of achieving an interference fit involves the

use of an interference bushing. In this case, the bolt need have only

a sliding fit in the bushing. Excessive clearance between the bolt and

the bushing, however, tends to produce early fatigue failure of the

bushing.

The full benefits which are available through the use of interference fits are secured only if the degree of interference is maintained

by regular inspection and repair in service. This is particularly true

when hardened steel bolts are used in softer plate materials. The bolts

tend to brinell into the plate material and reduce the tightness of the

fit. This brinelling action can be minimized by using high clamping

forces which are achieved, in turn, by high preloading of the bolts and

by the use of more bolts.

A relatively new technique for improving fatigue strengths around

bolt holes in aluminum alloys is represented by the proprietary development known as "coining." In this technique* a die or rolling wheel is

used to form a concentric groove around the hole. The metal adjacent to

the hole is forced inward, creating a residual compressive stress field,

and it is usually necessary to ream out the hole to its original diameter

to permit insertion of the bolt. The residual compressive stresses can

produce rather dramatic improvements in fatigue life by reducing the

effective mean stresses under cyclic loading. This phenomenon is thus

seen to be akin to structural preloading and opposite to the stress redistribution effects which result from the use of interference bolts.

In order to obtain the most favorable increases in fatigue properties

it is necessary to employ the correct coining depth, coining diameter,

groove radius, and spacing between holes. With thin sheet materials

coining of only one side has been found best while with stock thicker

than about 0. 1 inch both sides should be coined.

It is of interest to note that it is not generally advisable to preload a structure containing coined holes, presumably because the combined

effect of the preload and the coining can easily exceed the optimum amount

of localized plastic deformation mentioned in the previous section.

*Not to be confused with the ordinary coining process in which images or

-KaT-ar'i-ers of a punch and die are impressed onto a plane metal surface.
- 25 -

4.6 Fretting

(Supplementary references: 13, 20, 40, 71, 72)

Fretting is a phenomenon which is brought about by a combination of

(1) a high stress parallel to a surface which is in contact with a second

surface, and (2) relative movement at the point of contact. When both of

these conditions exist, a process of local welding and direct adhesion

takes place, followed by a tearing apart of the temporary connection so formed. Some investigators believe that eventual failure results fr<Mn

the adhesion and its accompanying stress concentrations rather than from

the surface disruption.

In the fatigue loading of a bolt and nut combination, relative m,ovement is most pronounced on the thread flanks but the maximum stresses are

in the thread roots. Hence, tests show that fretting is not ordinarily a contributor to fatigue failures in bolts.

On the other hand, many fatigue failures of joints are initiated by

fretting. The elimination of relative movement between the faying surfaces

by tight clamping is, therefore, very important. This represents another

argument for distributing bolts all over the contact area, using a close

pitch and preloading the bolts with a high torque. Indeed, to achieve

greater clamping, it has been suggested that the fatigue strength of a joint can be raised by increasing the bolt size even though failure does

not occur in the bolt and even though the cross sectional area of the

joint is thereby reduced.

When relative movement cannot be entirely eliminated, it is necessary

to lubricate the faying surfaces to delay fretting failure. Grease is a good preventative but not permanent. Molybdenum disulfide, either alone

or in an epoxy resin base, is more effective. Still another practice

involves the insertion of a thin wafer of a non-metallic material between

the contacting surfaces.

An extreme case of fretting has been observed in the repeated impacting of two stressed surfaces, such as a bolt shank and a hole wall. Lubrication was found to be ineffective here since a protective film could not

be maintained under the repeated impacts.

5. EFFECTS OF TEMPERATURE ON THE FATIGUE STRENGTH

OF BOLTS AND BOLTED JOINTS

(Supplementary references: 32, 45, 73 through 82)

Most specifications for aircraft fasteners are written around room

temperature properties in spite of the fact that aircraft operating

temperatures have been rising steadily over the past two decades. That
- 26 -

this procedure has not, perhaps, caused serious difficulty except in

exploratory vehicles may be attributed to the fact that materials have

been available which are capable of withstanding the temperature levels

attained by conventional military and conmercial airplanes in the past

without excessively deleterious effects. In the next decade, however,

airframe temperatures may be expected to severely try both the capabilities of materials and the ingenuity of designers.

Figure 5 presents an estimate of the maximum equilibrium skin

temperatures which will be reached by conventional manned military aircraft during the next ten years. Transient peak temperatures attained

will be even higher.

The problems introduced by operation at these temperatures are

multifold. For the purposes of this report, however, the most important

is the fact that the fatigue properties of materials do not vary with

temperature in any generally systematic or theoretically predictable

manner, particularly when stress concentrations are present. Furthermore, the results of fatigue tests at elevated temperatures are dependent upon the cycling speed employed. Hundreds of tests must be conducted

on each material before its fatigue properties as a function of temperature and time can be considered defined. Unfortunately, this can only

be considered a beginning toward the solution of hot fatigue problems.

Even a full set of elevated temperature fatigue properties for fasteners

and structural materials is insufficient, by itself, to properly design

a joint for a hot fatigue environment because, when temperature is

introduced as a variable, structural interactions take place between a bolt and its nut, and between the bolt-nut combination and the material

being fastened. These interactions are enumerated in the subsections

below. Further discussion of temperature effects is presented in

Section 6.

5.1 Bolts

Under elevated temperature exposure three structural interactions

take place between a bolt and the bolted material, all of which change

the amount of preload in the bolt. The first, and most significant of

these, is stress relaxation. Stress relaxation of bolts under creep

environments is a well-known, though perhaps not so we 11 -understood,

phenomenon. Frequently ignored, however, is the fact that relaxation

of the bolted material also reduces the bolt preload. Temperature

dependent changes of the elastic moduli of the bolt and the bolted material also change the magnitude of the preload in the bolt. For elevated

temperature applications, this change is almost always a reduction.

Finally, the development of differential thermal expansions between the

bolt and the bolted material causes changes in the bolt preload.
- 27 -

The latter Interaction can be minimized by choosing bolt and bolted

materials which have reasonably similar thermal expansion properties.

Failure to do this may result in either an immediate loss of preload

upon exposure to high temperature or an immediate increase in preload to

stress levels which may be high enough to cause rapid relaxation and the

eventual loss of .preload upon return to normal temperatures

Thermal expansion compatibility %rill also reduce the danger of

thermal fatigue of bolts in applications where temperatures are cycled

either frequently or severely.

Some bolts for aircraft engines are now made with the thread diameters

reduced, usually 0.003 in. from standard, for applications over 550* P.

It is claimed that this facilitates retightening, removability and

reusability which might otherwise become impossible due to the galling

created by elevated temperature oxidation and breakdown of the plating.

Figure 6 shows current thinking with regard to suitable bolt

materials for elevated temperature use. The temperature capabilities

represent a compromise between fatigue, creep and short-time properties,

and therefore are not directly applicable to any specific application.

Thread rolling may be difficult with some of these materials, and

the costs of this process may be high if special equipaent is needed and

if production quantities are small. On the other hand, tests indicate

that the residual stress benefits of rolled threads decrease with

temperature, so that requirements for rolled threads may, perhaps, be

eliminated in special cases if a more economical means of obtaining a smooth surface finish is found.

5 . 2 Nuts

When self -locking nuts are used, a further reduction of bolt preload occurs at elevated temperature from the relaxation of the locking

element regardless of whether it is of the Insert, beam or elliptical

type. This relaxation may be reduced somewhat by increasing the nut

thread length so that the imlt stresses in the locking device are re- duced. Relaxation of the locking element limits the number of times a nut may be satisfactorily retlghtened on a bolt to restore the desired

preload. Using a torque test similar to that described in MIL-N- 25027, one investigator found that self-locking nuts exposed to 900* F could

be reused at least 15 times, while those exposed to 1200* F had practically no reusability. The periodic replacement of self -locking nuts is,

therefore, a necessity.

A mitigating effect of elevated temperature exposure is that creep

in the threads, if not excessive, acts to distribute the bolt load among

the threads more uniformly, and thereby tends to prolong fatigue life.
- 28 -

other things being equal.

Hhen applying nuts to reduced diameter bolts (Section 5.1), it

must be recognized that different torques are required to produce given

amounts of preload because the friction characteristics are changed.

A graphic presentation of the approximate temperature capabilities

of nuts fabricated frcm various materials is given in Fig. 7.

5.3 Platings

Cadmium melts at 609° F and the peak useful temperature of cadmium

platings for short-time service is about 550° F. For continuous duty

the limiting temperature is reduced to about 450° F to prevent diffusion

of the cadmitim into the base metal. Diffused nickel -cadmium platings,

on the other hand, are satisfactory up to about 900° F. For still

higher temperatures, silver plating must be employed. Silver is a very

satisfactory plating material except for its high cost because it acts

as a good lubricant as well as a protective coating without the addition

of a supplementary dry film lubricant. The usefulness of silver, however, ends at about 1300° F. Beyond that temperature, a natural finish

is most coimnon although a hard chrome finish is occasionally employed

up to 1600° F, despite its deficiencies in terms of fatigue strength. A

proprietary finish called KAF is also reported by its manufacturer to be

usable to 1600° F, and two other products, Chromalloy W-2 and Durak MG,

have seen service at even higher temperatures.

5.4 Joint Materials and Designs

The selection of a configuration for a particular joint is generally

dictated by the structural application, rather than by the environment.

Hence, skin-to-spar cap connections, for example, are necessarily re- quired to be lap type joints, whether for ordinary or elevated temperature

use. Similarly, the principles governing the design of the bolt pattern

and the joint proportions, which were outlined earlier, apply here.

If the two materials being joined do not have comparable thermal

expansion properties, the already unequal distribution of load among the

various bolts becomes aggravated, and the peak loads taken by the end

bolts becomes greater. This is an important consideration in the selection

of doubler materials. However, just as creep tends to distribute a bolt

load more uniformly among the threads, it also tends to distribute the

total joint load more uniformly among the various bolts. Excessive creep,

of course, produces sufficient hole elongation to create undesirable slippage under alternating loads. Under these conditions, an interference fit

between a bolt and its hole, which might otherwise be used to combat

slippage, may be difficult to maintain.
- 29 -

Figure 8 presents an estimate of the capabilities of various structural materials for supersonic airframes. As in the case of Figs. 6 and

7, the capabilities shown represent compromises of various mechanical

properties. The optimum material for a particular application can only

be selected after a detailed study of the requirements. Also to be

considered is cost. The tabulation below indicates the approximate

costs of some structural materials. Actual costs vary with the requirements of the material specifications involved.

Alloy Relative Cost

302 X

17-7 PH 2x

A286 3x

Inconel X 6x

Titanium 20x

Rene 41 25x

S-816 30x

Beryllium 700x

6. DISCUSSIONS OF THE FATIGUE OF BOLTS

6.1 On the Evaluation of Bolts for Various Applications

Bolts occupy an unusual place in the field of aircraft components.

The gradual development of many aircraft components has been the result

of efforts on the part of both the aircraft industry and the component

manufacturers. By contrast, the tremendous amount of specialized equipment and fabricating know-how that is required to produce top quality

bolts has relegated their development primarily to the bolt manufacturer,

albeit with a view toward satisfying aircraft requirements. Of necessity,

therefore, many aircraft bolt procurement specifications are patterned

after the bolt maker's own fabrication specifications with the addition

of some acceptance tests. The acceptance tests, in turn, are designed

to guarantee uniformity in accordance with some standard of quality, but

usually do not serve to verify the mechanical properties which the

purchaser expects the bolt to have.

The ever -increasing cost of high performance military aircraft,

however, has forced the bolt user to reappraise the situation, and it is

inevitable that he must tighten his specifications in order to insure

reliability. At the same time, the specifications must be realistic;

they cannot exceed the bolt maker's capabilities. They must, therefore.
- 30 -

be based upon critical evaluations of available products.

The remainder of this subsection deals with the evaluation of bolts

in terms of fatigue requirements. Section 6.2 extends the discussion to

the specifications themselves.

a. Bolts for tension fatigue applications

The evaluation of - the capabilities of a bolt for a tension fatigue

application is based on many considerations but the most important is,

of course, the S-N curve for the bolt. For this reason, the author was

mildly surprised to find that there does not exist (or there is, at

least, not readily available) a compilation of S-N curves for the

various types and sizes of aircraft bolts. Clearly, the existence of

such a manual would have gone a long way toward making the present

report unnecessary. It is strongly recommended that the preparation of

such a manual be initiated immediately, and that it be supplemented and

revised regularly to keep it up to date. The procedure for preparing

such a manual could take the following form:

(1) Compile the data on fatigue of aircraft bolts that are

already available in the literature. A considerable

amount of such data does exist and a reference list

for this was started during the course of the survey

reported herein.

(2) Request the manufacturers and large users of aircraft

bolts to make available the data of this type which

they have secured for their own purposes

(3) Establish standard fatigue testing methods for evaluating aircraft bolts. In the long run, this is the

most important step. An excellent precedent has been

set by the National Aircraft Standards Committee of

the AIA with the issuance of specification NAS 1069,

"Tension Fatigue Test Procedure for Aeronautical

Fasteners" (see also ref. 47), which is more complete

thsin the "comparable" sections of Fed. Test Method

Std. No. 151.

(4) Set up a low-cost, open ended test program to evaluate

promising new bolts, new bolt features and new bolting

accessories as they become available. Examples of the

evaluations that would be immediately desirable might

include aircraft quality bolts with threads having 90** flank angles and with truncated threads, and nuts

having the so-called Equa-Stress thread form.
- 31 -

Naturally, the availability of S-N curves for aircraft bolts is not

the whole answer. The effects of preload should also be known, at least

as to whether it is beneficial to the particular bolt or bolt material

in question. This, incidentally, points up a shortcoming in NAS 1069,

which calls for the minimimi load to be 10 percent of the maximum load

in all cases, and thus furnishes no clue at all regarding the effects

of preload.

b. Bolts for non-tension fatigue applications

In the past, tensile fatigue problems in bolts have overshadowed

shear fatigue problems , shear fatigue being more critical in the sheet

than in the bolt. However, a fairly uniform characteristic of most

high temperature bolting materials is their relatively low shear strength.

Consequently, it appears wise to prepare now for a potential shear

fatigue problem in bolts.

Virtually nothing is known about fatigue in bolts loaded in "pure"

single or double shear. It is even debatable whether fatigue failure

can result from such loading. This question is purely academic, however,

since in actual practice a certain amount of bending almost always

accompanies so-called shear loading of bolts. Bending loads, of course,

are very damaging to threaded members in terms of fatigue life. Just

how damaging they are cannot be estimated from tension fatigue data

with present knowledge.

Clearly, research is needed to provide additional knowledge on this

subject. This additional knowledge may demonstrate a relationship between

shear/bending fatigue and tension fatigue properties. However, it is

more probable that no such relationship will evolve, in which case it

will be necessary to establish standard shear fatigue test methods for

evaluating aircraft bolts comparable to the tension methods already discussed. The importance of this research and development cannot be over- emphasized. There already exists a dire need for interaction curves describing the fatigue properties of aircraft bolts under combined loads.

c. Bolts for elevated temperature fatigue applications

Most of the comments made in the preceding paragraphs also apply

to the evaluation of bolts for hot fatigue applications. The compiling

of S-N curves, in this case for various temperatures, is particularly

important. There is no dependable way to analytically evaluate the

fatigue properties of a new bolt at a given temperature from the S-N

curve at room temperature. The feasibility of experimentally evaluating

the fatigue properties of bolts at elevated temperatures, on the other

hand, has been demonstrated, ref. 32, at least for tension loadings.
- 32 -

Having determined the elevated temperature fatigue properties of a bolt, it is then important to examine its stress relaxation properties.

It is entirely possible that certain potential bolt materials may exhibit

excellent basic fatigue properties at elevated temperature, but at the

same time have such poor stress relaxation properties that rapid loss

of preload and premature fatigue failures in service would be unavoidable. Standard bolt stress relaxation test methods, therefore, must

also be developed.

6.2 On the Specification of Fatigue-Resistant Bolts

a. Bolts for use at ordinary temperatures

Most procurement specifications for aircraft bolts, such as MIL-B-6812, do not call for any acceptance fatigue tests. It goes

without saying that such bolts should not be employed in fatiguesensitive areas. A few of the higher strength bolts, such as the MS20004 series, do

require acceptance fatigue tests. The test conditions for these, which

are prescribed in MIL-B-7838, call for tests under a single specified

stress amplitude. The question is, "Are the results of tests under a single stress amplitude sufficient to substantiate an entire S-N curve?"

The answer is a qualified "yes", at least insofar as tension fatigue

alone is concerned. The qualifications are as follows:

(1) Bolts fabricated in accordance with each of the permissible variations given in the specification should

all have S-N curves which are similar to, or superior

to, an arbitrary S-N curve which it is desired to

substantiate. (The permissible variations in most

specifications extend to a choice of several materials and fabrication processes.)

(2) The prescribed fatigue stress amplitude for the

acceptance tests should be chosen after careful

study of the S-N curve, to be sure that it is in a region near the middle of the curve where the

dS/dN slope is relatively high and the scatter in

N is relatively low.

As mentioned earlier, the present state of knowledge precludes the

use of acceptance tests in tension fatigue as a substantiation of shear

fatigue properties

b. Bolts for elevated temperature use

Separate procurement specifications for fatigue-resistant bolts

will have to be prepared for each temperature range (e.g. 900, 1200,
- 33 -

1500° F) because the materials Involved for each rangg have different

characteristics

The procedure of using a single fatigue stress amplitude to substantiate an entire S-N curve is probably as applicable to the elevated

temperature case as it is to the room temperature one. However, the tests

will have to be conducted at the temperature in question because it is

highly unlikely that bolts fabricated from a ntraiber of materials will

have similar S-N curves both at room temperature and the temperature in

question. An alternative procedure would limit each bolt specification

to a single material (or, perhaps, two similar ones) and a single

sequence of manufacturing operations. This procedure has been adopted

in specifications such as AMS 7476 and AMS 7478. In this case, room

temperature fatigue tests can provide reasonable substantiations for

elevated temperature S-N curves providing the material does not experience

significant metallurgical instability over the range of temperatures

involved. The addition of an elevated temperature test would furnish

greater reliability.

This is only the beginning, however. As pointed out in Section 5,

the elevated temperature fatigue properties of a bolt, per se, do not

define its fatigue properties in a joint at elevated temperatures

because of the interactions which take place between a bolt, its nut,

and the bolted material. Limitations on the usage of the bolt, such

as permissible bolt-nut-material combinations, dimensional ratios, and

temperature ranges should be called to the attention of the user. These

limitations could be presented in the form of an AND standard, or included in a docinnent such as MIL-HDBK-5 (formerly ANC-5) . The disadvantage of this approach is that it is too restrictive, at least in the case

of MIL-HDBK-5. Each deviation from the suggested usage would require

approval from the procuring agency. A more fitting approach, at least

for the present, would be to include these criteria in the respective

bolt procurement specifications. The inclusion of usage limitations

in bolt procurement specifications is common, and several now specify

the appropriate nuts to be used.

The task of charting these criteria is primarily a matter of computation at this point, since much of the required data on stress relaxation

and elastic and thermal properties are available in the literature. These

criteria, with little modification, would also be applicable toward

minimizing the possibility of thermal fatigue failure.

Eventually, of course, material of this nature should go into

MIL-HDBK-5. It may be a while, however, since this dociraient does not

yet recognize thie existence of such phenomena as stress relaxation and

thermal fatigue.
- 34 -

6.3 On Errors in Fastener Replacement

In Section 1 , it was pointed out that the variations of aircraft

bolts commercially available are becoming more and more numerous. Under

this situation, the possibility of using an incorrect bolt in a specific

location must be recognized. The inadvertent use of an MS9033 bolt

instead of an NAS 624 bolt, for example, could lead to disastrous consequences. The likelihood of such an error occurring is apparently greater

when fasteners are being replaced than when they are being initially

installed. Thus, elevated temperatures multiply the possibilities of

danger in the case of nuts which require regular replacement. The

accidental use of the wrong nut in a high temperature region could lead

to rapid loss of preload and premature fatigue failure.*

The straightforward solution to this problem, namely, the indoctrination of repair, maintenance and inspection personnel to exercise

particular care in this respect has not been entirely successful and a need for supplementary measures is indicated. Three approaches of a supplementary nature are outlined below. These may be implemented

either singly or simultaneously, by directive from the procuring agency

or voluntarily by the organizations involved.

a. Decrease the chances of installing a wrong bolt

The problem is not concerned with the inadvertent use of bolts with

the wrong head style, or of the wrong size, but of "look-alike" bolts

having different mechanical properties. Hence, this approach calls for

a reduction in the number of "look-alike" bolts which shall be acceptable

for use in assemblies fabricated for the procuring agency. The pruning

of the vindesirables could be effected after a comparative study of the

mechanical properties, including the S-N curves, of the various types.

As an example, MS20008-19 bolts can replace all AN149080 and NAS 148-32

bolts without any sacrifice of structural integrity. The penalty for

this approach is that the user pays a premium for mechanical properties

which he does not always need. In the example cited, the premium would

be about seven percent. On the other hand, the resulting greater use

of the MS bolt would tend to lower its price and, further, some savings

would undoubtedly accrue to the user from a reduction in his inventories.

Another aspect of the problem deals with the wrenching recesses in

countersunk head bolts (see Section 3.5). Research is required to

establish the relative merits of the many recesses now available, and

*It is recognized that a similar problem exists with the tools used for

fastener replacement. Use of the wrong tools, particularly those of

the impact type, has caused serious structural damage on occasion.
- 35 -

steps should then be taken to standardize on the more desirable ones.

b. Decrease the chances of failure resulting from

the installation of a wrong bolt

This means upgrading the mechanical properties of the lower quality

bolts insofar as this is economically feasible. One way to do this was

brought out in this report. It was pointed out that rolled threads

give fatigue strengths which are superior to those obtained with cut

threads; that thread rolling is an economical process when large quantities are involved; and that most bolt procurement specifications do not

necessarily require that threads be rolled. It is suggested that specifications not requiring thread rolling, such as MIL-B-6812 and MIL-B-7874,

be amended to require it.

c. Increase the ease with which the presence of an improper

bolt in an assembly may be detected

The usual technique of marking bolts and nuts with numbers and

symbols requires too close an inspection for rapid identification. It is

recommended, therefore, that the advantages of color coding be exploited

more fully than heretofore. At present, this, marking technique is used

rather infrequently (cf . , MIL-N-25027) , despite its promise. Indeed, the

enormous success which this technique has had in the electronics industry

could well be a goal to strive for in the preparation and revision of

fastener specifications.

Another suggestion is the promotion of an industry-wide practice of

placing markings on assemblies adjacent to each bolt pattern, the markings

to correspond with those on the proper bolts and nuts to be used.

* * *

The projects which led to this report were conducted under the

sponsorship and with the financial assistance of the Bureau of Naval

Weapons. The author is indebted to many persons for valuable suggestions

and for information not readily available in the literature. Among them

are M. S. Rosenfeld of the Naval Air Material Center, C. P. Baum,

A. P. Cowles, H. Klug and G. D. Norman of the Bureau of Naval Weapons,

and J. A. Bennett, L. K. Irwin, Miss W. D. Kroll and J. I. Price of the

National Bureau of Standards.
- 36 -

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by coining, Am. Soc. Mech. Engrs . Preprint 61-AV-35 (1961).

71. Field, J. R. , Fretting corrosion on a screwed joint under prolonged

fatigue loading, Engineer (London) 200 , p. 301 (1955).

72. Walker, P. B., Fretting in the light of aircraft experience, J. Roy.

Aeron. Soc. 63, p. 293 (1959).

73. Dolan, T. J., Problems of metallic fatigue at high temperature.

Metal Progress, p. 97 (April 1952).

74. Gatewood, B. E., Thermal stresses, pp. 5-8 (McGraw-Hill Book Co.,

Inc., New York, N. Y., 1957).

75. Nat. Aircraft Standards Comm. , Aircraft Industries Assoc, Report

of the high-strength high- temperature materials for standard

parts symposium, Dallas (Nat. Standards Assoc, Washington, D.C.,

1957).

76. Silwones, S. S., and R. A. Degan, Test results guide hightemperature design of bolted assemblies, Product Eng. ^, No.

12, p. 79 (1957).

77. Anon., Esna high temperature design catalog No. 5918, Elastic Stop

Nut Corp. of America, ADL 5918 (1959).

78. Christensen, R. H., Fatigue design requirements for future air and

space vehicles, Proc Symposium Fatigue Aircraft Struct., U. S.

Air Force Wright Air Development Center Tech. Rep. 59-507, p. 776

(1959) .

79. Anon., 1960 aerospace forecast of technical requirements (Aerospace Industries Assoc, Washington, D. C, 1960).

80. Wagner, H. J., Recent developments in superalloys. Defense Metals

Information Cjent^tr Memo . 64 (1960).

81. Stanton, V.'A., Stainless and superalloy spring materials, Proc.

Chicago Assn. Spring Manufacturers Convention (I960))

82. Anon., Space/aeronautics research & development handbook 1960-

1961 (Conover-Mast Publications, Inc., New York, N. Y., 1960).

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Fig. 4 Double shear joints with (a) stress -relieving

grooves, (b) reduced plate thickness.
I960

Year

Fig. 5 Temperature trend for manned military

aircraft in continuous flight.
Alloy Temperature, ®F

H-ll

422 MOD.

I9-9DL —

INCOLOY 901

INCONEL X - AF 1753

INCONEL 700

WASPALOY

UDIMET 500

o 2

o o O

o o lO

0) o OJ ro to

A286

^

DISCALOY

^

M 252

R - 235

^

Fig. 6 Approximate temperature capabilities of

commercially available aircraft bolts.
AF 1753

HASTELLOY

M 252

UDIMET 500

o O O

o in

CVI ro lO

Alloy Temperature, ®F

O o

O lO

0) o

CHROMALLOY

^

422 MOD. »-

347 FM

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N-155

^

Fig. 7 Approximate temperature capabilities of

commercially ovoilible aircraft nuts.
Alloy Temperature, ®F

o o o 8

O

o in o

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AM-350 »-

PH I5-7M0 ^

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N-155

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AF 1753

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^

RENE 41 —

Fig. 8 Approximate temperature capabilities of

several airframe materials.

II. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAJU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at. its major laboratories in Washington, U.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineering in the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

front cover.

WASHINGTON, D.C

Electricity. Resistance and Reactance. Electrochemistry. Electrical Instruments. Magnetic Measurements.

Dielectrics. High Voltage.

Metrology. Photometry and Colorimetry. Refractometry. Photographic Research. Length. Engineering Metrology.

Mass and Scale. Volumetry and Densimetry.

Heat. Temperature Physics. Heat Measurements. Cryogenic Physics. Equation of State. Statistical Physics.

Badiation Physics. X-^^ay. Radioactivity. Radiation Theory. High Energy Radiation. Radiological Equipment.

Nucleonic Instrumentation. Neutron Physics.

Analytical and biorganic Chemistry. Pure Substances. Spectrochemistry. Solution Chemistry. Standard Refer- ence Materials. Applied Analytical Research.

Mechanics. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Rheology. Combustion

Controls. Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion. Metal Physics. Elec

trolysis and Metal Deposition.

Mineral ftodDCts. Engineering Ceramics. Class. Refractories. Enameled Metals. Crystal Growth. Physical

Properties. Constitution and Microstructure. Building Research. Structural En^neering. Fire Research. Mechanical Systems. Organic Building Materials. Codes and Safety Standards. Heat Transfer. Inorganic Building Materials. Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics, Operations Research. Data Processing Systems. Components and Techniques. Computer Teohnology. Measurements Automation.

Engineering Applications. Systems Analysis.

Atomic Physics. Spectroscopy. Infrtu-ed Spectroscopy. Solid State Physics. Electron Physics. Atomic Physics,

tostmmentation. Engineering Electronics. Electron Devices. Electronic Instrumentation. Mechanical Instro- ments. Basic Instrumentation. Physical Chemistry. Thermochemistry. Surface Chemistry. Organic Chemistry. Molecular Spectroscopy. Molecular Kinetics. Mass Spectrometry.

Office of Weights and Measnes.

BOULDER. COLa

Ciyogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Cryogenic Technical Services. Ionosphere Research and Propagation. Low Frequency and Very Low Frequencv Research. Ionosphere Research.

Prediction Services. -Son-Eartb Relationsh^s. Field Engineering. Radio Warning Services. Vertical Sounding Research. Radio Propagation Engineiering. Data Reduction instrumentation. Radio- Noise. Tropospheric Measurements.

Tropospheric Analysis. Propagation-Terrain Effects. Radio-Meteorology. Lower Atmosphere Physics.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. Radio and Microwave Materials. Atomic Frequency and Time Interval Standards. Electronic Calibration Center. Millimeter-Wave Research.

Microwave Circuit Standards. Radio Systems, .^iplied Electromagnetic Theory. Hi|^ Frequency and Very High Frequency Researeh. Modulation

Research. Antenna Research. Navigation Systems.

I^iper Atmospliere and Space Physics. UpcNcr Atmosphere and Plasma Physics. Ionosphere and Exosphere

Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.